1. Technical Field
The present invention relates to electric rotating machines that are used in, for example, motor vehicles as electric motors and electric generators. In addition, the invention can also be applied to industrial machines and household electrical appliances.
2. Description of Related Art
There are known electric rotating machines which include an Interior Permanent Magnet (IPM) rotor. The IPM rotor includes a rotor core that has a plurality of slots formed therein and a plurality of permanent magnets each of which is inserted in a corresponding one of the slots of the rotor core.
However, during high-speed rotation of the rotor, stress concentration may occur, due to centrifugal force, at those portions of the rotor core which surround the corner portions of the permanent magnets.
To solve the above problem, Japanese Patent Application Publication No. 2002-436008 discloses a technique, according to which recesses are provided in the inner surfaces the slots of the rotor core so as to reduce stress concentration in the rotor core.
More specifically, as shown in FIGS. 5A and 5B, each of the permanent magnets 101 has a rectangular cross section perpendicular to the axial direction of the rotor core 107 (i.e., perpendicular to the paper surface of FIGS. 5A and 5B), and thus has four corner portions. The four corner portions include a corner portion 103A which is located radially innermost in the four corner portions and adjacent to a bridge portion 102 of the rotor core 107, and a corner portion 103B which is located radially outermost in the four corner portions and diagonally opposite to the corner portion 103A. The bridge portion 102 extends radially between an adjacent pair of the slots 105 of the rotor core 107. For each of the corner portions 103A and 103B of the permanent magnet 101, a recess 106 is formed in the inner surface of the corresponding slot 105 (i.e., the inner surface of the rotor core 107 which defines the corresponding slot 105) in close proximity to the corner portion, so as to be recessed in a reference direction of the permanent magnet 101. Here, the reference direction denotes a direction which is perpendicular to both the magnetization direction of the permanent magnet 101 and the axial direction of the rotor core 107. At those areas A of the rotor core 107 which are in close proximity to the corner portions 103A and 103B of the permanent magnet 101 and circled with dashed lines in FIG. 5B, it is generally easy for stress concentration due to centrifugal force to occur. However, by providing the recesses 106 in the inner surface of the corresponding slot 105, it is possible to reduce stress concentration due to centrifugal force at the areas A.
Further, each of the permanent magnets 101 has such a property that it contracts in the reference direction when heated and expands in the reference direction when cooled.
Accordingly, in the case that the permanent magnets 101 are inserted into the corresponding slots 105 of the rotor core 107 through a process during which the permanent magnets 101 are heated and the electric rotating machine is later used at a lower temperature, each of the permanent magnets 101 will be expanded in the reference direction to press the inner surface of the corresponding slot 105.
However, according to the technique disclosed in Japanese Patent Application Publication No. 2002-136008, for each of the permanent magnets 101, the corner portions 103A and 103B of the permanent magnet 101 are arranged in contact with the inner surface of the corresponding slot 105 in the reference direction of the permanent magnet 101. Consequently, when the permanent magnet 101 is expanded in the reference direction, the corner portions 103A and 103B of the permanent magnet 101 will press the inner surface of the corresponding slot 105 in the reference direction, thereby causing stress concentration due to thermal stress to occur at the areas A.
That is, Japanese Patent Application Publication No. 2002-136008 failed to disclose or even address how to prevent stress concentration due to thermal stress from occurring at the same areas A of the rotor core 107 as stress concentration due to centrifugal force.
On the other hand, Japanese Patent Application Publication No. H9-294344 discloses a technique for preventing cracks from occurring at the corner portions of the permanent magnets due to centrifugal force.
More specifically, as shown in FIG. 6, for each of the corner portions 103 of the permanent magnets 101, there is formed a recess 116 in the inner surface of the corresponding slot 105 around the corner portion 103, thereby preventing the corner portion 103 from making contact with the inner surface of the corresponding slot 105.
However, Japanese Patent Application Publication No. H9-294344 also failed to disclose or even address how to prevent stress concentration due to thermal stress from occurring at the same areas of the rotor core as stress concentration due to centrifugal force.